1. Field of the Invention
The invention relates to power transmission belts and, more particularly, to a method of manufacturing a rubber sheet for incorporation into a power transmission belt and made up of at least two different rubber compositions which are combined using an extrusion process.
2. Background Art
Short fibers are commonly embedded in rubber to define component portions of power transmission belts. The fibers are typically oriented so that their lengths are substantially parallel. To establish the orientation of the fibers, it is known to mix fibers in a non-cured rubber. The rubber and fiber mix is then placed between a pair of calendar rolls which are operated at different rotational speeds as is typical of a rolled sheet fabrication process. The length of the fibers align with the rolling direction for the resulting sheet, which can then be cut to dimensions to fit around a molding drum. A plurality of these sheets are then overlapped and laminated to produce a composite sheet with a thickness dictated by a final desired belt configuration. The composite sheet is then wrapped around a molding drum, with the length of the fibers oriented widthwise/axially of the drum, preparatory to forming a power transmission belt in conventional manner.
More specifically, a V-ribbed belt or raw-edged V-belt may be conventionally formed by initially wrapping one, or a plurality of, sheets of cover fabric/canvas around a cylindrically-shaped molding drum. A cushion rubber layer is in turn wrapped around the cover canvas sheet(s) after which at least one load carrying member is spirally wrapped against the cushion rubber layer. The load carrying member is made commonly in the form of an elongate cord. The compression rubber layer is then wrapped in place to complete a sleeve preform. In the preform, the compression rubber layer may have a thickness that is made up of three to four rolled sheets formed by the above-described method, with the lengths of the short fibers oriented in the widthwise direction. The resulting preform is then cured, after which the preform is processed to produce individual power transmission belts.
To establish a consistent, parallel, orientation of the fibers, the thickness of the sheets must be maintained below a predetermined dimension, which is normally significantly less than the end thickness of the compression rubber layer, or other layer into which the fibers are incorporated. The combining of the individual sheets to produce the desired end thickness inevitably adds time and labor costs to the manufacturing process, that would otherwise not be encountered if the desired end thickness for the sheet could be achieved in a single step.
One proposed solution to this problem has been to employ a cylindrically-shaped mold/die through which a rubber composition, incorporating the fibers, can be extruded, as shown in JP-B-9847. More particularly, the mold has an expansion portion which is used to orient the lengths of the fibers to extend in a circumferential direction relative to cylindrical inner and outer peripheral mold surfaces which bound a flow passage through which the rubber composition is extruded. The mold has a middle space between an inlet space and an outlet space. The volume of the middle space is larger than that of the inlet space. The cross-sectional area of the outlet space is made larger by a predetermined amount than the cross-sectional area of the inlet space. The width of the inlet portion of the flow passage is smaller than that of the middle portion, with the width of the outlet portion being not greater than the width of the middle portion.
As shown in JP-A-8-74936, it is also known to form a sheet from a rubber composition including short fibers using an expansion mold/die and an extruder which is located at an outlet portion thereof. The outlet portion has V-shaped molding grooves. A cylindrically-shaped, ribbed rubber product is formed, which can be strategically cut to allow reconfiguration thereof into a sheet form. The resulting sheet is formed into a continuous sleeve shape around a molding drum, cured, and ultimately ground to produce ribs, as on a V-ribbed belt.
Commonly, a particularly sticky material, such as chloroprene rubber, is used to make sheets using the conventional expansion die technology. The sticky rubber composition may not flow smoothly against the mold surfaces due to a high coefficient of friction between the rubber composition and the mold surfaces. This condition may cause a roughening of the rubber surfaces and separation of the fiber from the matrix rubber and/or a random, nonuniform distribution of fibers therein. The resulting sheet may be of a sufficiently poor quality that it may not be practically used, for example, in a compression rubber layer of a power transmission belt.